Various patents and other publications are referred to throughout the specification. Each of these publications is incorporated by reference herein, in its entirety.
As a complex and sensitive organ of the body, the eye can experience numerous diseases and other deleterious conditions that affect its ability to function normally. Many of these conditions are associated with damage or degeneration of specific ocular cells, and tissues made up of those cells. As one example, diseases and degenerative conditions of the optic nerve and retina are the leading causes of blindness throughout the world. Damage or degeneration of the cornea, lens and associated ocular tissues represent another significant cause of vision loss worldwide.
The retina contains seven layers of alternating cells and processes that convert a light signal into a neural signal. The retinal photoreceptors and adjacent retinal pigment epithelium (RPE) form a functional unit that, in many disorders, becomes unbalanced due to genetic mutations or environmental conditions (including age). This results in loss of photoreceptors through apoptosis or secondary degeneration, which leads to progressive deterioration of vision and, in some instances, to blindness (for a review, see, e.g., Lund, R. D. et al. 2001, Progress in Retinal and Eye Research 20: 415-449). Two classes of ocular disorders that fall into this pattern are age-related macular degeneration (AMD) and retinitis pigmentosa (RP).
AMD is the most common cause of vision loss in the United States in those 50 or older, and its prevalence increases with age. The primary disorder in AMD appears to be due to RPE dysfunction and changes in Bruch's membranes, e.g., lipid deposition, protein cross-linking and decreased permeability to nutrients (see Lund et al., 2001 supra). A variety of elements may contribute to macular degeneration, including genetic makeup, age, nutrition, smoking and exposure to sunlight.
RP is mainly considered an inherited disease—over 100 mutations have been associated with photoreceptor loss (see Lund et al., 2001, supra). Though the majority of mutations target photoreceptors, some affect RPE cells directly. Together, these mutations affect such processes as molecular trafficking between photoreceptors and RPE cells and phototransduction, for example.
Other less common, but nonetheless debilitating retinopathies can also involve progressive cellular degeneration leading to vision loss and blindness. These include, for example, diabetic retinopathy and choroidal neovascular membrane (CNVM). Diabetic retinopathy may be classified as (1) non-proliferative or background retinopathy, characterized by increased capillary permeability, edema, hemorrhage, microaneurysms, and exudates, or 2) proliferative retinopathy, characterized by neovascularization extending from the retina to the vitreous, scarring, fibrous tissue formation, and potential for retinal detachment. In CNVM, abnormal blood vessels stemming from the choroid grow up through the retinal layers. The fragile new vessels break easily, causing blood and fluid to pool within the layers of the retina. Damage or progressive degeneration of the optic nerve and related nerves of the eye constitutes another leading cause of vision loss and blindness. A prime example is glaucoma, a condition of the eye that is made up of a collection of eye diseases that cause vision loss by damage to the optic nerve. Elevated intraocular pressure (IOP) due to inadequate ocular drainage is a primary cause of glaucoma, but it can also develop in the absence of elevated IOP. Glaucoma can develop as the eye ages, or it can occur as the result of an eye injury, inflammation, tumor, or in advanced cases of cataract or diabetes, or it can be caused by certain drugs, such as steroids. The primary features of the optic neuropathy in glaucoma include characteristic changes in the optic nerve head, a decrease in number of surviving retinal ganglion cells, and loss of vision. It has been proposed that a cascade of events links degeneration of the optic nerve head with the slow death of retinal ganglion cells observed in the disease, and that this cascade of events can be slowed or prevented through the use of neuroprotective agents (Osborne et al., 2003, Eur. J. Ophthalmol. 13 (Supp 3): S19-S26).
Cellular damage and degenerative conditions also affect other parts of the eye. For example, cataracts result from gradual opacification of the crystalline lens of the eye. It is believed that once begun, cataract development proceeds along one or more common pathways that culminate in damage to lens fibers. This condition is presently treated by surgical removal and replacement of the affected lens. Another example concerns the cornea and surrounding conjuctiva that make up the ocular surface. The limbal epithelium, located between the cornea and the bulbar conjuctiva, contains corneal epithelial stem cells. Limbal epithelial cell deficiency (LECD) is a condition that occurs, for example, in Stevens-Johnson syndrome and thermal or chemical burns. LECD often leads to an imbalance between the corneal epithelium and the conjunctival epithelium in which the cornea is covered by invading conjunctival epithelial cells, which severely compromises the corneal surface and affects visual acuity (Nakamura, T. & Kinoshita, S., 2003. Cornea 22 (Supp. 1): S75-S80).
The recent advent of stem cell-based therapy for tissue repair and regeneration provides promising treatments for a number of aforementioned cell-degenerative pathologies and other ocular disorders. Stem cells are capable of self-renewal and differentiation to generate a variety of mature cell lineages. Transplantation of such cells can be utilized as a clinical tool for reconstituting a target tissue, thereby restoring physiologic and anatomic functionality. The application of stem cell technology is wide-ranging, including tissue engineering, gene therapy delivery, and cell therapeutics, i.e., delivery of biotherapeutic agents to a target location via exogenously supplied living cells or cellular components that produce or contain those agents (For a review, see Tresco, P. A. et al., 2000, Advanced Drug Delivery Reviews 42: 2-37).
An obstacle to realization of the therapeutic potential of stem cell technology has been difficulty in obtaining sufficient numbers of stem cells. One source of stem cells is embryonic or fetal tissue. Embryonic stem and progenitor cells have been isolated from a number of mammalian species, including humans, and several such cell types have been shown capable of self-renewal and expansion, as well differentiation into a variety of cell lineages. In animal model systems, embryonic stem cells have been reported to differentiate into a RPE cell phenotype, as well as to enhance the survival of host photoreceptors following transplantation (Haruta, M. et al., 2004, Investig. Ophthalmol. Visual Sci. 45: 1020-1025; Schraermeyer, U. et al., 2001, Cell Transplantation 10: 673-680). But the derivation of stem cells from embryonic or fetal sources has raised many ethical issues that are desirable to avoid by identifying other sources of multipotent or pluripotent cells.
Adult tissue also can yield stem cells useful for cell-based ocular therapy. For instance, retinal and corneal stem cells themselves may be utilized for cell replacement therapy in the eye. In addition, neural stem cells from the hippocampus have been reported to integrate with the host retina, adopting certain neural and glial characteristics (see review of Lund, R. L. et al., 2003, J. Leukocyte Biol. 74: 151-160). Neural stem cells prepared from fetal rat cortex were shown to differentiate along an RPE cell pathway following transplantation into the adult rat subretinal space (Enzmann, V. et al., 2003, Investig. Ophthalmol. Visual Sci. 44: 5417-5422). Bone marrow stem cells have been reported to differentiate into retinal neural cells and photoreceptors following transplantation into host retinas (Tomita, M. et al., 2002, Stem Cells 20: 279-283; Kicic, A. et al., 2003, J. Neurosci. 23: 7742-7749). An ocular surface reconstruction in a rabbit model system, utilizing cultured mucosal epithelial stem cells, has also been reported (Nakamura, T. & Kinoshita, S., 2003, supra). While these reports show promise for the use of adult progenitor and stem cells in cell-based therapy for the eye, it must be noted that adult stem cell populations are comparatively rare and are often obtainable only by invasive procedures. Further, adult stem cells may have a more limited ability to expand in culture than do embryonic stem cells.
Thus, a need exists for alternative sources of adequate supplies of cells having the ability to support, augment or replace lost cellular function in the eye. A reliable, well-characterized and plentiful supply of substantially homogeneous populations of such cells would be an advantage in a variety of diagnostic and therapeutic applications in ocular repair and regeneration, including drug screening assays, ex vivo or in vitro trophic support of ocular and other useful cell types, and in vivo cell-based therapy.